1. Field of the Invention
The present invention is directed to a computed tomography apparatus, and in particular to a computed tomography apparatus of the type for conducting a spiral-helical scan.
2. Description of the Prior Art
Standard computed tomography systems include a housing, referred to as a gantry, which has support elements therein supporting a measuring unit, also known as a scan frame, which includes an x-ray radiator and a radiation detector. The scan frame is rotatable around an examination region so that a subject disposed in the examination region is trans-irradiated by x-rays from the x-ray radiator from a number of successive angular directions (projections). The subject lies on a patient table, in the examination region. From the output signals generated by the detector during a scan, a computer generates an image of a slice of the examination region (i.e. a slice of the patient), which is displayed on a monitor.
In medicine, minimally invasive surgical interventions for diagnosis and therapy are being increasingly conducted with CT (computed tomography) monitoring. So-called spiral helical scan computed tomography is frequently employed for this purpose which, in addition to the above-described operation, employs a longitudinal displacement of the patient table with the patient thereon, so that an image of a larger volume of the subject can be obtained. Spiral-helical CT has thus made a step from simple slice imaging to volume imaging, and therefore it is possible, instead of making a slice exposure for monitoring the position of a surgical instrument, to make three-dimensional exposures (volume exposures) for reliable positioning of surgical instruments in a critical region of the subject. The anatomotical details in this critical region may vary from patient to patient, thus making such volume imaging particularly useful for such interventional purposes.
German Utility Model 92 18 322 discloses an apparatus for invasive therapy employing computed tomography monitoring, with the computed tomography apparatus being adjustable relative to a patient bed along a longitudinal direction. This known system can be employed only in combination with one patient bed, i.e., a patient bed that is specifically configured to interrelate with the other components of the computed tomography system. German Utility Model 77 30 503 discloses a mobile x-ray apparatus which is provided for producing x-ray exposures at the hospital bed of a patient. The production of computed tomograms with this mobile apparatus is not possible.